Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 262000

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
300 PM CDT FRI AUG 26 2016

300 PM CDT

Through Saturday...

An air mass we have become quite familiar with this mid-late of high moisture and modest to high
instability...will return late tonight into Saturday bringing with
it thunderstorm and heavy rainfall potential. This pattern is
unfortunately somewhat messy, which makes it challenging for
specific storm timing/coverage, especially into the day Saturday.

A surface warm front is draped from west to east across the mid-
section of Illinois this afternoon and will begin to acceleratenorthward
through tonight in response to a short wave lifting northeast from
the Central Plains. Gradual moist advection amid broad scale
ascent north of the warm front should result in isolated showers
developing this evening over northern Illinois. As a 30 kt low-
level jet develops into eastern Iowa, convection should blossom to
the immediate west of the CWA. While some of the moisture
transport nose will be pointed into northern Illinois, it is
somewhat uncertain how quickly storms will move into the area. The
greater deep layer moisture convergence and short wave-induced
lift is late tonight into the early morning hours Saturday.
Continue the highest PoPs during that time frame. Convective
allowing models show a plethora of solutions, not surprising
given this type of air mass, but an ensemble approach such as from
the SSEO favors late tonight especially in north central and far
northeast Illinois.

Increasing shear tonight will support some storm organization, but
the threat for any severe storms remains small. Precipitable
waters near two inches and deep warm cloud depths as observed on
the morning ILX sounding will expand over the area and once
again support very efficient rainfall rates in any of the
overnight and Saturday activity. The collaborative thoughts are
the highest QPF will be over north central Illinois tonight with
some locations possibly realizing around two inches or higher
depending on duration. The greater moisture convergence is just to
our west, but will be something to monitor, and confidence is not
enough at this time for any type of flood watch.

For Saturday, much will depend on influence of early morning
activity. Do expect there to be a high amount of morning cloud
cover along/behind the warm front lifting northward. As the warm
sector expands in there should be a decent amount of afternoon
destabilization. Some confluence/focus is noted in the model low-
level wind fields, but again tough to say where exactly that may
end up due to influence of morning rain. Maintain a chance of
afternoon storms, with a little higher noted central/eastern CWA.
The deep layer shear/instability would certainly be enough to
support some severe storms with any afternoon development, as well
as locally very heavy rainfall.



306 AM CDT

Sunday through Friday...

East coast upper ridge will gradually flatten Sunday through
Tuesday, but in the meantime, expect active weather pattern to
continue through Tuesday as northern Illinois and northwest
Indiana remain under a moist and unstable airmass. Forcing
becomes rather nebulous through this timeframe, and anticipate
mesoscale features may end up driving diurnally favored
convection. A lake breeze is expected each day and may be one
trigger in particular to keep an eye on. Expect high temperatures
to be in the mid 80s each day, Sunday through Tuesday, except
along the lake front where lake breezes will limit heating.

For the latter half of the week, upper ridge will amplify across
the mid section of the country which will dislodge cooler air
and allow it to spill into portions of the Great Lakes. Strong
high pressure is progged to build into Ontario with northerly flow
bringing a glancing blow of 850 mb temps in the low teens locally.
There are timing differences between the models, with the ECMWF
bringing the cold air down faster resulting in mid to upper 70s
for highs as early as Wednesday while the GFS is a considerably
slower. Did not make any changes from the blended consensus
forecast, but there is certainly potential that the current
forecast may be a several degrees too warm. Will keep an eye on
coming model runs for better agreement before making any big



For the 18Z TAFs...

The challenges with the Chicago area TAFs are timing/duration of
showers and possible storms tonight into Saturday as we return
into a moist and unstable air mass.

High pressure across Minnesota through far northern Illinois will
continue a light northeast to east wind this afternoon with a
potential lake breeze push offering 8-10 kt easterly winds at ORD
and MDW.

Warm and moist advection aloft will steadily increase tonight,
with northward-moving isolated/scattered light showers expected
to develop over northern Illinois this evening. The more
pronounced focus for well as some storms...will
arrive overnight into the early morning hours Saturday as a
weather disturbance lifts northeast into Wisconsin. The confidence
in showers reaching the Chicago airports has grown, as well as the
idea of isolated/scattered TSRA in the northern Illinois region.
Confidence in duration of precip is fairly low, as it could be
just an hour or possibly three or four hours. RFD is the most
likely area to see TSRA nearby, as well as IFR conditions during

In the wake of the warm front lifting northward Saturday morning,
MVFR clouds should linger with medium-high confidence, and cannot
rule out IFR clouds. Even some light fog may be present depending
on how quickly rain ends relative to daybreak. The chance for
scattered storms will return Saturday afternoon as the moist and
unstable warm sector spreads over the region.



239 AM CDT

Fairly benign conditions are expected across the lake outside of
occasional thunderstorm periods. High pressure will build across
the lake today and east the pressure gradient across the north
half. A weak system will cross the north half of Lake Michigan
Saturday night with southeast winds increasing some ahead of it.
High pressure will return to the eastern Great Lakes region Sunday
into early next week. Southerly winds will redevelop as low
pressure slides east across North Central Canada. This low will
send a cold front across the northern half of Lake Michigan.
Modest high pressure will rebuild across Ontario behind the front,
and this will maintain north to northeast flow across Lake






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